The web part templates work the same way, and follow the same rules, as the templates for the ASP.NET Membership controls the web parts are built on. Microsoft documents how to edit those templates, and which fields are required, here:
ASP.NET Membership control templates (MSDN)
The Create User Step template also requires the FirstName, LastName and Email fields.
Where the default templates live
/_layouts/##/FBA/WEBPARTS
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\##\TEMPLATE\LAYOUTS\FBA\WEBPARTS
## is your SharePoint version number.
Making your own template
The easiest way to start is to copy the default template and edit the copy however you like. The required fields above must stay in the template. Any other field can be removed.
When your template is ready, point the web part to it in the web part properties.